This invention relates generally to nuclear reactors and more particularly, to methods and apparatus for cleaning jet pump nozzles in a nuclear reactor.
A reactor pressure vessel (RPV) of a boiling water reactor (BWR) typically has a generally cylindrical shape and is closed at both ends by a bottom head and a removable top head. A top guide typically is spaced above a core plate within the RPV. A core shroud, or shroud, typically surrounds the core and is supported by a shroud support structure. Particularly, the shroud has a generally cylindrical shape and surrounds both the core plate and the top guide. There is a space or annulus located between the cylindrical reactor pressure vessel and the cylindrically shaped shroud.
In a BWR, hollow tubular jet pumps positioned within the shroud annulus provide the required reactor core water flow. An inlet mixer, an upper portion of each jet pump, is laterally positioned and supported against two opposing rigid contacts within restrainer brackets by a gravity actuated wedge. The inlet mixers are each held in place at the top end by a preloaded beam. A jet pump diffuser, the lower portion of each jet pump, discharges water from the inlet mixer into the lower core plenum to maintain adequate water flow.
At least some known inlet mixers include a pre-nozzle section; a plurality of nozzles circumferentially distributed about an inlet mixer axis and a plurality of inlet vents. Each nozzle includes an inner surface and an outlet. The inlet vents are circumferentially distributed about the inlet mixer axis adjacent the nozzle outlets. As jets of water exit the nozzles water from the annulus is drawn into the inlet mixer via the inlet vents, where it is mixed with water from recirculation pumps.
During reactor operation, a scale forms on inner surfaces of the jet pump nozzle. This scale buildup is a significant problem because it causes a loss of cooling flow and reduces reactor output, which increase costs. Further, maintenance in the annular volume between the core shroud and the reactor pressure vessel, in which the inlet mixers are located, is difficult because of limited access and radiation levels.
In one aspect, a cleaning device is provided for a jet pump nozzle in a nuclear reactor. The cleaning device includes a tubing section including a first end, a second end and a mid-section between the first end and the second end. A pole adapter is coupled to the tubing section at the mid-section, and a hydrolaze head assembly is coupled to the tubing first end.
In another aspect, a cleaning assembly is provided for a jet pump nozzle in a nuclear reactor. The cleaning assembly includes a high-pressure pump, a high-pressure hose coupled to the high-pressure pump, a fluid control valve configured to regulate fluid flow through the high-pressure-hose, a handling pole, and a cleaning device coupled to the high-pressure hose downstream of the fluid control valve. The cleaning device includes a tubing section, a pole adapter, and a hydrolaze head assembly. The tubing section includes a first end, a second end and a mid-section between the first end and the second end, with second end coupled to the high-pressure hose. The pole adapter is coupled to the tubing section at the mid-section and is configured to releasably attach to the handling pole. The hydrolaze head assembly is coupled to the tubing section first end.
In another aspect, a method for cleaning scale from a jet pump nozzle in a reactor pressure vessel, utilizing a cleaning assembly is provided. The cleaning assembly includes a handling pole connected to a cleaning device that includes a hydrolaze head assembly having a rotating sleeve. The method includes positioning the hydrolaze head assembly in the reactor pressure vessel, providing high-pressure fluid to the cleaning device, positioning the hydrolaze head assembly within the jet pump nozzle, expelling high pressure fluid from the hydrolaze head assembly to remove scale from the jet pump nozzle, and repositioning the hydrolaze head assembly vertically in the jet pump nozzle.